1. Field of the Invention
The present invention relates to a fuel cell.
2. Description of the Related Art
In mobile information devices, drastic advances have been made in recent years, in terms of effecting miniaturization, weight reduction, and higher functionality. Also, with the development of information devices and apparatuses, advances have been made steadily in terms of effecting miniaturization, weight reduction, and higher capacity of cells as electric sources for their use.
The most popular electric power source for driving the current mobile telephone devices is a lithium ion cell. The lithium ion cell had a high driving voltage and a high cell capacity at the onset of the practical application, and the performance has been improved to keep pace with the development of the mobile telephone devices. However, there is a limitation to the performance improvement of the lithium ion cells, and it is becoming more and more difficult for the lithium ion cells to satisfy requirements (a higher service capacity, for example) as a power source for driving mobile telephone devices for which demand for a higher functionality is expected to increase more and more in future.
Under such a situation, hopes are concentrated on the development of new electric power generation devices which can substitute for lithium ion cells. A fuel cell is one of the new electric power generation devices. A fuel cell is a device for generating electricity through generating electrons and protons by supplying a fuel to the negative electrode, and reacting the protons with oxygen supplied to the positive electrode.
The most significant feature of this system is that a long-time continuous electricity generation is possible through supply of a fuel and oxygen, and accordingly, can be applied to electric power sources for various devices in the same way as the secondary cells by refueling instead of charging of the secondary cells. Because of this, developments on the fuel cell are progressing actively not only for dispersed power sources and for large-scale generators for the electric car use, but also for ultracompact electricity generating units for notebook-size personal computers and mobile telephones.
In the field of small fuel cells particularly, investigations are actively progressing on fuel cells for which an aqueous methanol solution is used as a fuel. At present, Nafion, a perfluorosulfonic acid polymer from du Pont is a mainstream material for a solid electrolyte for use in such a small fuel cell. However, Nafion membranes are known to have a high affinity for alcohol for use as a fuel, and have high swelling characteristics. As a result, the fuel penetrates into the air electrode (positive electrode) side, causing problems of decrease in the output density and electric generation capacity. The very high price of Nafion is also a factor which prevents fuel cells from being more popularized. Hence, expectations are growing for inexpensive materials that can substitute for Nafion.
As a method for solving such problems, it was reported that engineering plastics are subjected to sulfonation with fuming sulfuric acid or the like in order to form inexpensive electrolyte membranes with a restricted fuel crossover (fuel permeation) {see, for example, Japanese Unexamined Patent Application Publication No. H06-49202 (claims)}. However, with this method, it is difficult to control the rate of sulfonation of an electrolyte membrane, and it is also difficult to control the swelling characteristics of an electrolyte formed.
As a method for controlling the swelling characteristics together with the mechanical strength, methods by means of polymer blending and copolymerization are investigated {see, for example, Japanese Unexamined Patent Application Publications No. H06-76838 (paragraph 0016, 0017), and No. H6-275301 (paragraph 0006)}. However, it is the principal objective of these methods to increase the mechanical strength, and accordingly, it is not very effective for restricting fuel crossover of the electrolyte membrane, and higher ion conductivity can hardly be expected.
A structure given by chemically cross-linking a polymer is disclosed as a method for controlling the swelling characteristics {see, for example, Japanese Unexamined Patent Application Publications No. 2003-217365 (claims)}. With this method, it is possible to introduce a cross-linked structure. However, it is difficult to selectively decide positions to be cross-linked, and therefore, it is thought to be impossible to make an electrolyte membrane with uniform properties as a whole.